This invention relates in general to a method of repositioning teeth for use in orthodontic treatment. Particularly, this invention relates to the use of orthodontic appliances for producing tooth movements. More particularly, this invention relates to the use of a plurality of elastic repositioning appliances for producing such tooth movements.
Orthodontic treatments involve repositioning misaligned teeth and improving bite configurations for improved cosmetic appearance and dental function. Repositioning teeth is accomplished by applying controlled forces to the teeth over an extended period of time. This is conventionally accomplished by wearing what are commonly referred to as xe2x80x9cbraces.xe2x80x9d Braces comprise a variety of appliances such as brackets, bands, archwires, ligatures, and O-rings. After they are bonded to the teeth, periodic meetings with the orthodontist are required to adjust the braces. This involves installing different archwires having different force-inducing properties or by replacing or tightening existing ligatures. Between meetings, the patient may be required to wear supplementary appliances, such as elastic bands or headgear, to supply additional or extraoral forces.
Although conventional braces are effective, they are often a tedious and time consuming process requiring many visits to the orthodontists office. Moreover, from a patient""s perspective, they are unsightly and uncomfortable. Consequently, alternative orthodontic treatments have developed. A particularly promising approach relies on the use of elastic positioning appliances for realigning teeth. Such appliances comprise a thin shell of elastic material that generally conforms to a patient""s teeth but is slightly out of alignment with the initial tooth configuration. Placement of the elastic positioner over the teeth applies controlled forces in specific locations to gradually move the teeth into the new configuration. Repetition of this process with successive appliances comprising new configurations eventually move the teeth through a series of intermediate configurations to a final desired configuration. A full description of an exemplary elastic polymeric positioning appliance is described in U.S. Pat. No. 5,975,893, and in published PCT application WO 98/58596 which designates the United States and which is assigned to the assignee of the present invention. Both documents are incorporated by reference for all purposes.
In addition to their ease of use, polymeric positioning appliances are generally transparent, providing an improved cosmetic appearance, and impart substantial force on the teeth, due to stiffness of the appliance. The stiffness of an elastic positioning appliance is a result of the modulus of the thermoformable polymer materials from which it is made. The higher the modulus of the materials, the higher the stiffness of the appliance. When a patient positions such an appliance over a prescribed group of teeth, one or more of the teeth will provide a base or anchor region for holding the positioning appliance in place while the stiffness of the polymeric material will impart a resilient repositioning force against one or a portion of the remaining teeth. However, the stiffer the appliance, the more difficult it is to slip the misaligned appliance over the teeth and fully engage the appropriate surfaces; the appliance often has the tendency to disengage or xe2x80x9cpop offxe2x80x9d. Likewise, once it is firmly seated, it is more difficult to remove. Further, a stiff appliance is less forgiving in cases of lowered patient compliance. If a patient were to remove the appliance for an unprescribed period of treatment time, the patient""s teeth may move slightly out of the planned tooth arrangement. When attempting to reapply the appliance, it may be too rigid to accommodate these slight differences and a new appliance may need to be created. Similarly, the tooth positions defined by the cavities in each successive appliance must not differ beyond a limiting dimension from those defined by the prior appliance or, again, it may be too rigid to accommodate the differences. Consequently, only small increments in tooth repositioning may be made with each appliance.
Thus, it would be desirable to provide tooth positioners, systems, and methods which apply adequate force to selected teeth yet overcome the inherent limitations of stiffness in the polymeric material. Likewise, it would be desirable to reduce the number of positioners required for a treatment plan by increasing the size of the repositioning increments throughout the plan. Further, it would be desirable to reduce the cost of lowered patient compliance by reducing the need for new appliances to be created for patient treatment resumption. At least some of these objectives will be met by the designs and methods of the present invention described hereinafter.
The present invention provides improved devices, systems and methods for repositioning teeth from an initial tooth arrangement to a final tooth arrangement. Repositioning is accomplished with a system comprising a series of polymeric appliances configured to receive the teeth in a cavity and incrementally reposition individual teeth in a series of successive steps. This is accomplished by applying force to specific surfaces of the teeth to cause directed movement. In order to apply such force, one or more of the teeth will provide a base or anchor region for holding the positioning appliance in place while the stiffness of the polymeric material will impart a resilient repositioning force against one or more of the remaining teeth. However, such stiffness creates limitations in ease of use, patient compliance, and overall cost in material, manufacturing labor and treatment time.
To overcome these limitations, the present invention utilizes polymeric or other material appliances with portions differing in rigidity, hardness, or stiffness. Portions of the appliance designed to apply specific forces may have different elastic moduluses (stiffnesses) and/or hardnesses than other portions. Alternatively, elastic moduluses and/or hardnesses may vary from one appliance to the next in a successive series to accomplish various treatment goals. Thus, the systems and methods of the present invention provide the design, production and use of such multiple stiffness positioning appliances in orthodontic treatment. Similarly, the devices of the present invention provide variable stiffness appliances which may be used independently for purposes other than repositioning, such as for retaining teeth in a desired position. Thus, reference hereinafter to repositioning appliances with portions having differing or varying stiffnesses or hardnesses is not intended to limit the scope of the present invention and is understood to include appliances of the described design for other purposes.
In a first aspect of the present invention, an elastic repositioning appliance may be comprised of portions with differing elastic moduluses. Elastic modulus may be used to express or describe the stiffness of a material or a material""s resistance to elastic deformation. Therefore, elastic modulus may be used hereinafter to refer to stiffness. The different portions of the appliances will also usually vary in hardness. More usually, stiffer portions will be harder while the less stiff portions will be softer. Hardness is usually measured as a xe2x80x9cdurometerxe2x80x9d reading on either the A or the D scale. In most instances, the present invention will be more concerned with the elastic modulus of the material since that will effect the force applied to the teeth for either moving the teeth or for gripping or anchoring the teeth. In other instances, however, the hardness of the material may be more important, e.g., to avoid trauma to soft tissue regions engaged by the appliance. The remaining description and claims generally refer to materials having greater and lesser sniffinesses. It will be appreciated that such terminology will also comprise materials having greater and lesser hardnesses.
The elastic modulus of a material is the ratio of the increment of unit stress to an increment of unit deformation within the elastic limit. When a material is deformed within the elastic limit, the bonds between adjacent atoms are stretched but not broken. The magnitude of the elastic modulus is indicative of the atomic and molecular bonding forces. When the stress is relieved, the material returns to its original shape and the deformation is nonpermanent. Different materials may have different elastic moduluses based on their molecular structures. Some materials, such as certain polymers, may be specially produced to have different elastic moduluses while retaining similar chemical compositions (and thus assuring compatibility of the different modulus materials in a single structure). Likewise, the elastic modulus of a polymer or other material may be enhanced or otherwise modified. This may be achieved by adding a powder, such as CaCO3, talc, TiO2, glass, diamond or a polymer powder, to name a few. In addition, this may be achieved by embedding structural reinforcements, such as metal pieces, strips, wires, mesh, lattices, networks, polymeric filaments, or the like. In addition, the elastic modulus may be altered by post-production methods, such as layering, coating, interpenetrating, treating with various chemical agents, and altering the temperature, to name a few. In the resulting appliance, the elastic moduluses of the varying portions will usually range from 0.5 to 5 GigaPascal (GPa), although in some instances portions of the appliance may fall outside of this range. The elastic modulus of one portion may differ from another portion by 25% to 600%, or more.
The differing elastic moduluses of different portions of the dental appliance shells of the present invention will exist while the device is present over teeth in a normal oral environment. Thus, different portions of the appliance shell will impart different forces to the immediately underlying teeth, where the level of the force depends both on the device geometry or tooth positions (relative to the underlying tooth or teeth, which may vary over time) and on the elastic modulus of that portion of the device (which will remain constant over time in the normal oral environment). The present invention should be distinguished from that described in copending application Ser. No. 09/250,962, where the stiffness of a dental appliance shell may change over time by expose to a non-oral environment, such as elevated temperature or changed osmolality. Of course, the dental appliance shells of the present invention which have different portions with differing stiffness may also incorporate regions (including the entire appliance) where a change in stiffness may be induced according to the teachings of application Ser. No. 09/250,962, the full disclosure of which is incorporated herein by reference.
In a first embodiment, portions of the shell of an elastic repositioning appliance may be composed of material(s) which differ in elastic moduluses and/or hardnesses along a mesial-distal axis. A mesial-distal axis may be defined as an axis following the gingival line or dental arch. Thus, the elastic repositioning appliance may be comprised of portions with a lower elastic modulus covering the molars, for example, and portions with a higher elastic modulus covering the remainder of the teeth. In this example, the portions may be relatively large so that a portion may receive one or more teeth, such as contiguous molars. This may be utilized when one or more teeth are to provide an anchor or base region for imparting repositioning force against another tooth or teeth. The portion of the appliance covering the anchor teeth may be of a relatively flexible nature with a lower elastic modulus than the portion covering the teeth to be repositioned. This is because the portions covering the anchor teeth may not need to apply repositioning forces to the teeth they cover; they may merely be designed to hold the appliance in place. Consequently, a high level of rigidity or stiffness may not be required. However, it may be appreciated that portions covering anchor teeth may in fact require a higher stiffness material than other portions, including portions which are designed to apply repositioning forces. Thus, any variation of stiffness or elastic modulus along a mesial-distal axis is included in this embodiment.
The introduction of such portions or regions with more flexibility provides utility in ease of use for the patient. The patient may find ease in positioning the appliance with the more flexible portions first which may guide the appliance in placement of the more rigid, slightly misfit portions designed for repositioning. This sequence may be reversed in removal of the appliance. Likewise, such flexibility may also allow for any slight differences in mold versus appliance versus dentition geometry which may otherwise make placement and removal of the appliance more difficult. In some cases, a generally misfit appliance may xe2x80x9cpop offxe2x80x9d or have a tendency to disengage even when properly positioned over the teeth. Increased flexibility may reduce these tendencies.
In further embodiments, portions of the elastic repositioning appliance may vary in elastic moduluses along different and/or additional axes. For example, moduluses may vary along a facial-lingual axis. Facial may be defined as next to or toward the lips or cheek, including terms labial and buccal. Lingual may be defined as next to or toward the tongue. Thus, a facial-lingual axis may be described as an axis following a radial or similar line from the tongue toward the lips or cheek and vice versa. Likewise, moduluses may vary along a gingival-crown axis. This may be described as a substantially vertical axis following a line from the top of the crown at the edge of the occlusal surface of a tooth toward the gingival line or root and vice versa. In a preferred embodiment, an appliance may have a portion with a lower elastic modulus covering the occlusal surfaces of the teeth and a portion with a higher elastic modulus covering the remaining surfaces of the teeth. Thus, the moduluses may vary along a facial-lingual axis and/or a gingival-crown axis, depending on the boundaries of the delineated portions. Such a design may incorporate added flexibility to the appliance while maintaining adequate repositioning forces in the most efficient areas.
In addition to varying in stiffness along the axes described above, the appliances of the present invention may vary in stiffness or hardness over the xe2x80x9cthicknessxe2x80x9d of the appliance. Usually, such variations and stiffness over the thickness will be accomplished by layering the device, i.e., with layers of differing stiffnesses or hardnesses being placed successively over the mold used to form the appliances, as described in more detail below. Thus, the appliances may comprise shells having first and second portions, as generally described above, where each of those portions comprise layers in a laminar structure. Usually, at least one of the first and second portions will comprise a continuous layer along the mesial-distal axis. The second and optionally additional layers may also be continuous along the mesial-distal axis, but will often be discontinuous, i.e., broken into two or more segments. Such layered devices can provide a variety of benefits. For example, layers formed from stiffer or harder materials can be used to more firmly engage teeth, while the less stiff or softer layers can be used to provide compliance and greater elasticity. In a particular preferred embodiment, the appliance comprises a discontinuous inner layer and a continuous outer layer. At least a portion of the inner layer is configured to engage individual teeth or groups of teeth and will be stiffer or harder than the outer layer. The outer layer, which is less stiff and therefore more compliant, provides the elasticity to move the teeth relative to one another, while the harder inner layer firmly engages the teeth to provide a better grip or anchor upon the teeth.
It may be appreciated that the elastic modulus of the appliance shells may vary over any number of delineated portions. Such portions may be of any size, shape, thickness, or dimension. Thus, such portions may receive entire teeth or they may be of the size to cover only a portion of a tooth or dental surface. When portions are relatively large, an appliance may be divided into, for example, two to five portions. Portions adjacent to one another differ in elastic moduluses, however not all portions of an appliance may differ from each other, such as in the case of an appliance with portions alternating between two moduluses. When portions are relatively small, an appliance may contain an unlimited number of portions, varying along any axis or combination of axes.
In a second aspect of the present invention, such appliances comprised of portions having differing stiffness may be used independently or in a series with similar or differing devices. When used independently, the appliance may be worn to achieve a specific goal with a single device. For example, the appliance may be used as a xe2x80x9cretainerxe2x80x9d to hold the teeth in a desired position. Or, the appliance may be used for a specific one-time repositioning movement, such as xe2x80x9cfinishingxe2x80x9d or correcting a slight misalignment. When used in a series, the appliances may comprise a system for repositioning teeth from an initial tooth arrangement to a final tooth arrangement. In this case, a plurality of incremental elastic position adjustment appliances comprising polymeric or other material shells are successively worn by a patient to move teeth from one arrangement to a successive arrangement. Individual appliances may be configured so that their tooth-receiving cavity has a shape or geometry corresponding to an intermediate or end tooth arrangement intended for that appliance. Thus, successive individual appliances may have a shape or geometry differing from that of the immediately prior appliance. According to the present invention, some or all of the individual appliances may also be comprised of a material stiffness differing from the stiffness of the immediately prior appliance. In addition, each individual appliance be comprised of portions with varying stiffnesses. In some cases, of course, individual appliances in the system may not vary in stiffness from prior or successive appliances, but only in geometry. In other cases, individual appliances may vary only in stiffness (and not in geometry) when compared to immediately prior or subsequent appliances. Thus, systems according to the present invention may be comprised of appliances having stiffness varying within the appliance and/or from one appliance to the next in the series.
In a specific embodiment, a system of elastic repositioning appliances may comprise individual appliances having uniform elastic moduluses over their entire tooth contact area where the moduluses will differ among successive appliances used in a course of treatment. The elastic modulus of a given appliance may be chosen to be most suitable for a specific type of tooth movement, such as translating, tipping, root uprighting, rotation, extrusion, intrusion or a combination of these. For example, translation may require 70-120 gm of force, whereas rotation may only require 35-60 gm of force. Therefore, an elastic positioning appliance designed for translating teeth may need to have a higher elastic modulus than one designed for purely rotating teeth. This is again due to the fact that stiffness of the appliance is a critical factor in imparting repositioning force. Consequently, a series of appliances may be produced for a treatment plan in which successive appliances designed for a specific tooth movement may all have substantially similar elastic moduluses. At the point in the treatment plan in which a different type of tooth movement is desired, further appliances designed for the new tooth movement may have substantially similar elastic moduluses to each other but different from the previous appliances. Such a sequence may be repeated at any time or may continue with new moduluses and tooth movements.
In an additional specific embodiment, one or more appliances may be produced with a suitably flexible elastic modulus to receive and resiliently reposition teeth from an unprescribed arrangement to a prescribed arrangement. This might be necessary in cases of lowered patient compliance. If a patient were to remove an appliance for an unintended and/or extended period of a prescribed treatment time, the patient""s teeth may move slightly out of the planned tooth progression. When attempting to reapply the appliance, an appliance which is too rigid may not be able to accommodate these slight differences. Thus, a more flexible appliance (but having an identical geometry) may be produced for this purpose and may be incorporated into the treatment plan at any given point in the series of successive appliances. The ability to return to the same geometry is an advantage because it minimizes the need to replan the treatment protocol.
In a third aspect of the present invention, systems for repositioning teeth from an initial tooth arrangement to a successive tooth arrangement comprise a plurality of incremental elastic position adjustment appliances in which at least one appliance has the same shape yet different elastic modulus as an immediately prior appliance. In a specific embodiment, a series of incremental appliances may be produced with differing elastic moduluses to reposition teeth from an initial tooth arrangement to the next successive tooth arrangement in a progression of arrangements to the final arrangement. Each of the appliances in the series from the first to the next successive tooth arrangement may have the same shape or geometry since the tooth movement represents one step in tooth movement. However, the variance in elastic moduluses may allow for a larger step or increment in tooth movement than may be obtainable with consistent, rigid appliances. For example, an appliance may be produced with a tooth arrangement which is substantially misaligned from the initial arrangement. High modulus appliances may not be flexible enough to allow the appliance to fit over the teeth in the initial arrangement. However, a series of appliances of the same shape may be produced with increasing elastic moduluses from relatively low to adequately high. The patient may begin with the lowest elastic modulus appliance which may be the most flexible to fit over the teeth. As the teeth are repositioned, the patient may successively utilize each appliance in increasing modulus until the teeth have conformed to the successive tooth arrangement. At that time, the patient may begin a new series of appliances with varying moduluses and a shape to reposition the teeth to the arrangement of the next step in the repositioning progression. The ability to reduce the number of different appliance geometries required for a single course of treatment can provide a significant reduction in planning effort and manufacturing costs.
In a fourth aspect of the present invention, the elastic modulus of an appliance or portions of an appliance may be modified in a number of different ways. To begin with, the elastic modulus may be determined by the choice of materials. For example, metals will generally have a higher elastic modulus than polymers due to atomic structure. For example, the modulus values for metals may range between 48 and 414 GPa, whereas the modulus for polymers may range from 0.5 to 35 GPa. Thus, it will be possible to form appliances having moduluses which differ greatly by forming different portions from metal(s) and polymer(s), or by forming successive appliances from metals and polymers. Usually, however, the appliances will comprise or consist of a polymeric shell formed from a single polymer, multiple polymers, copolymers, and the like, typically by thermoforming and/or lamination. Stiffness of a polymer may be varied within a range (typically 0.5 GPa to 5 GPa) by changing the molecular structure of the polymer chains. Polymer chains with hindered side-chains are unable to pack as closely as those with smaller side-chains. Thus, such a polymer may have more intermolecular motion and therefore a lower bulk elastic modulus. Stiffness can also be changed by controlling the degree of cross-linking as well as the cross-linking entity within a polymer or copolymer. Further, alternatively, differing elastic moduluses may be created within the same polymer shell by layering or laminating the same or different polymers. Two layers of a polymer material bonded together to form an integral appliance, i.e., an appliance having a monolithic shell structure where the layers are resistant to delamination, may have a higher elastic modulus than a single layer of such material. Thirdly, different elastic moduluses may be created with a single layer of one type of polymer material by production methods, such as coating, treating with various chemical agents, and altering the temperature, to name a few.
Further, different elastic moduluses may be produced by forming selectively reinforced and/or composite-type materials. For example, a polymer material may be reinforced with structures such as strips, wires, pieces, mesh, lattices, networks, and the like. These structures may be comprised of any suitable material, particularly metals and alloys but also including polymer filaments, wires, braids, and the like. Likewise, composite materials may be comprised of interpenetrating polymeric networks. An interpenetrating polymeric network is comprised of a base material and an additional material that interpenetrates the base material to alter its mechanical properties. For example, the base material (A) may be a solid polycarbonate. The added material (B) may be a liquid polymer, monomer or crosslinking agent which is allowed to interpenetrate and activate to form a composite network. The composite (A+B) may have a stiffness which is greater than the sum of its parts, (A) and (B). Further, another material (C) may also be allowed to interpenetrate and activate to form a new composite network. The composite (A+B+C) may also have a stiffness which is greater than the sum of its parts, (A), (B) and (C). With this method, any number of composites may be formed providing a wide range of mechanical properties, specifically stiffnesses. In addition, a number of these production methods may provide materials with gradual changes in elastic moduluses. For example, purposely irregular coating of a polymer material may provide higher stiffness in areas with thicker coating and lower stiffness in areas with thinner coating. This may be applied to a number of production methods.